1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer, a copying machine, or a facsimile machine and, more particularly to an image forming apparatus suitably designable as an inline-type image forming apparatus which has a plurality of electrostatic latent image bearing members, and in which images formed on the respective electrostatic latent image bearing members are successively transferred onto one intermediate transferring member or a transferring material in a superposition manner to form a multicolor image.
2. Related Art
Various color image forming apparatuses for forming a color image on a transferring material by using an electrophotographic recording system have been devised and some of them have been put to practice.
A representative example of such image forming apparatuses is a type of image forming apparatus which has one photosensitive body used as an electrostatic latent image bearing member, and a plurality of developing apparatuses respectively containing developers of a plurality of colors, and in which electrostatic latent images on the photosensitive body are successively developed by using the developing apparatuses. More specifically, rotary developing apparatuses integrally combined with developing apparatuses for development of four colors: yellow, magenta, cyan, and black are provided around one photosensitive body. Each of electrostatic latent images related to the colors and formed on the common photosensitive body is visualized as a toner image by the corresponding one of the developing apparatuses at a development position reached by the latent image with the rotation of the photosensitive body. Each time one toner image is obtained in this manner, it is transferred onto a transferring material, which is a recording member having a sheet shape such as a paper sheet. These steps are repeated to complete a multicolor image.
Another type of apparatus has also been devised in which toner images in different colors are selectively superposed on the surface of a photosensitive body to form a multicolor toner image on the photosensitive body surface, and the multicolor toner image Is thereafter transferred at a time onto the transferring material.
An inline-type of image forming apparatus different from those described above has also been proposed in which a plurality of photosensitive bodies are used as electrostatic latent image bearing members, and toner images in different colors are separately formed by developing apparatuses of each color respectively facing the photosensitive bodies and are successively transferred onto a transferring material while the transferring material is being conveyed to form a multicolor toner image. Each photosensitive body and other image forming means, including a developing apparatus and a charging device, are integrally combined into an image forming means with respect to each color (hereinafter referred to as xe2x80x9cprocess stationxe2x80x9d). Process stations thus formed are successively placed along the direction of movement of a transferring material conveying member for conveying the transferring material. There is also a type of image forming apparatus having a similar construction but using a different transferring system such that toner images in different colors are not directly transferred onto a transferring material but successively superposed on an intermediate transferring member to form a multicolor image, which is transferred onto a transferring material at a time. In many cases, each process station is provided in the form of a process cartridge detachably attachable to the image forming apparatus.
Each of the typical types of color image forming apparatus using inline electrophotographic recording systems has both advantages and disadvantages. However, from the viewpoint of the recent development of speedup techniques with the change of market needs, inline systems are considered to be more advantageous than others. Also, intermediate member transfer systems devised as transferring means have the advantage of adaptability to various kinds of transferring material. For this reason, various products using these systems are being designed and put to practice.
However, attempts are being eagerly made to achieve improvements in certain specification items, e.g., reductions in size, weight and power consumption with respect to the above-described color image forming apparatuses. There is also a trend toward machine constructions using components simplified in various respects.
As a method of developing an electrostatic latent image, two-component development methods using a mixture of a toner and a carrier and one-component development methods using a magnetic toner alone are generally known. However, use of a carrier and a need for a so-called ATR mechanism for adjusting the mixing ratio of a toner and a carrier in two-component development methods conflict with requirements for reductions in size and weight.
Non-magnetic one-component development methods disclosed in Japanese Patent Application Laid-open Nos. 58-116559, 60-120368 and 63-271371 attract attention as development methods free from the above-described problem. Non-magnetic one-component development methods require no ATR mechanism, use a simplified arrangement in which charge is caused by friction between a developer, a developer carrying member, and a layer thickness regulating means such as a blade in contact with the developer carrying member, and enable formation of a sharp vivid color image without high-temperature dark transfer failure such as that caused in the case of using a magnetic one-component developer. Therefore, non-magnetic one-component development methods have been used with favorable effects.
In such non-magnetic one-component development methods, a developer carrying member is coated with a developer by a layer thickness regulating means such as a blade, and the developer is charged by friction against the blade or the surface of the developer carrying member. However, if the thickness of the coat is increased, developer particles not sufficiently charged exist. Such developer particles are liable to cause fog and to scatter. Therefore, there is a need to regulate the thickness of the developer coating layer to a sufficiently small value, and it is necessary to maintain the blade in pressure contact with the developer carrying member at a sufficiently high pressure. The force received by the developer in this state is larger than that received by a developer in a two-component development method or a one-component development method using a magnetic toner. A non-magnetic one-component development method is also known which uses an elastic roller in place of a blade as a means for regulating the developer layer thickness on a developing roller provided as a developer carrying member. The elastic roller contacts the developing roller at an upstream position in the direction of rotation of the developing roller. The elastic roller has the function of scraping off toner left on the developing roller instead of being fed for development, and newly supplying toner onto the developing roller.
In the arrangement using the blade or the elastic roller, toner on the developing roller rubs on the blade or the elastic roller. As the toner undergoes a larger number of repeated cycles of rubbing, an externally added material adsorbed to the toner surface is liberated or embedded in the toner resin. Such a toner degradation phenomenon becomes considerable if the time period during which toner on the developing roller rubs on the blade or the elastic layer, i.e., the rotating time of the developing layer, is increased. In particular, in the latter half of the life of toner, image degradations, such as fog, a reduction in density, and a defect due to transfer failure, occur.
In the case of an inline-type image forming apparatus which forms a multicolor image, the photosensitive drums and the developing rollers in all of process stations provided as image forming means are driven during most of the printing time from pre-rotation at an image formation preparatory stage to post-rotation for a cleaning operation or the like after-image formation. The rotational speeds of the photosensitive drums and the developing rollers are not changed, and these rotating components continue rotating at the maximum speed for a long time. Therefore, this image forming apparatus is disadvantageous in terms of degradation of the developer in comparison with a below-described image forming system using rotary developing apparatuses including developing apparatuses for development of four colors, yellow, magenta, cyan, and black.
To prevent toner degradation, in an inline system, a method may be used in which the photosensitive drum and the developing roller in the process stations not operating for image forming (charging, development, transfer, etc.) in the printing time from pre-rotation to post-rotation are stopped. This method, however, maximizes the relative speed between the transferring material conveying member or the intermediate transferring member and the photosensitive drum, so that local rubbing of the surface of the photosensitive drum on the transferring material conveying member or the intermediate transferring member occurs during the stoppage period. As a result, the photosensitive drum surface is scratched by a slide friction on to cause image defects.
The above-described problems may be solved by a method in which the process stations not operating for image forming (charging, development, transfer, etc.) in the printing time are spaced apart from the transferring material conveying member or the intermediate transferring member, and the photosensitive drums and the developing rollers therein are stopped. This method, however, requires a spacing mechanism which is set in the image forming apparatus to enable each of the process stations in the image forming apparatus to be spaced from the transferring material conveying member or the intermediate transferring member independently of the others, resulting in an increase in size of the apparatus and an increase in manufacturing cost.
Further, the rotating time of the developer carrying member varies depending on paper passage modes. In a situation where the frequency of continuous printing is low, pre-rotation and post-rotation are frequently performed. As a result, driving time of the developing roller is increased. Therefore, a considerable image defect due to toner degradation may occur even during the nominal life, depending on operating conditions.
An object of the present invention is to provide an image forming apparatus which can be used for a long time without degradation of a developer.
Another object of the present invention is to provide an image forming apparatus in which the time period during which a developer carried on a developer carrying member rubs on another component is reduced.
Still another object of the present invention is to provide an image forming apparatus arranged to prevent shortening of the life of an image bearing member.
A further object of the present invention is to provide an image forming apparatus smaller in size and having a reduced manufacturing cost.
Still a further object of the present invention is to provide an image forming apparatus capable of changing the peripheral velocity of a developer carrying member between a development period and a non-development period.
These and other objects and features of the present invention will become apparent from the following detailed description of preferred embodiments of the present invention in conjunction with the accompanying drawings.